james_tg

Someone told me the other day that it is horsepower that allows you to accelerate fast. I thought for a second and realized that most people out there don't fully understand the physics, etc involved. Here it goes!

For all you car buffs out there that aren't quite sure of how things work:

horsepower = torque * rpm / 5252

--------------------------------------
1 HP = 0.745699872 kW
1 Kw = 1.341022089 HP

and

1 lb-ft = 1.35581795 N-m
1 N-m = 0.73756214 lb-ft

so,

HP = ft-lb * RPM / 5252
kW = N-m * RPM / 5252
--------------------------------------

Torque is what determines a vehicle's ability to accelerate. Torque is after all, the engine's ability to apply a turning force - it stands to reason that torque does thusly determine the ability to turn the wheels and overcome inertia.

Gearing simply takes advantage of the engine's ability to produce large RPM values which are "turned into" more torque, thus yeilding higher horsepower as a result. In fact, horsepower is relative to the relationship between torque and RPM.

Example:

engine #1 specs

- max torque: 10 pound-feet
- max rpm: 90'000

Engine #1 can produce 171.36 horsepower at 90'000 RPM.

engine #2 specs

- max torque: 10 pound-feet
- max rpm: 90

Engine #2 can produce 0.17 horsepower at 90 RPM.

Both engines produce 10 pound-feet of torque, however .. based on RPM alone, engine #1 can be expected to out-accelerate engine #2.

Thus,

Horsepower is simply a mathematical relationshop between torque and RPM. Horsepower would not be possible without torque.

another way of looking at it:

Horsepower is a conveniant method for comparing two engines without the bias of RPM and TORQUE differences.

So next time sombody says: "I'm putting out 170 horsepower at the wheels" ask: "That is great, but is this also at 90'000 rpm?"

punkindrublic

i knew that already


that forumla is not 100% accurate though

if you did a dyno run and just looked at the torque numbers... you would notice if you worked it out for hp you would notice the numbers would be lower....

which is because that formula doesn't account for tire bounce/pressure or clutch slip and a few other variables....


the is a more compicated formula using car weight, 1/4 time and rpm that will give you hp numbers.... that is what the gtec pro thing uses

james_tg

When was the last time you saw any reputable manufacturer using gTEC dyno numbers?

FYI. The formula is as close to exact as you can come without using calculus to determine the limits.

The "complicated formula" using vehicle mass, 1/4 time and RPM is even more flawed than a "normal" dynojet dynamometer. You have a LARGE error on mass for starters (when was the last time you weighed your car with YOU in it and the gas tank as full as the run you are testing?), your 1/4 time is likely FAR slower than it should be due to human error [poor launch - even top dragsters have very "mathematically" poor launches], and RPM measured by your tachometer is off by a large percentage too (because of the sensing system). Recall back to school that error multiplies!! This "method" has an average error percentage of +-20% or worse - that is a combined error rate of 40%.

Why do you think people pay big $$ to have their car put on an actual dynamometer as opposed to just using "the formula" ??

If you want more info on a real dynamometer, here is a nice model:

http://www.flyinmiata.com/tech/rototest.asp


fewer variables to account for = fewer errors

In short - what you are arguing is completely different from what I posted about. Please don't think that your gTEC numbers are even close to acceptable measurements - they simply aren't. For the closest to "true" measurement possible, you need to be tested on a rototest style dynamometer. Even "dynojet" style "rolling drum" dynos have a "correction factor" because of their design.

imported_gatherer

personally I only care about torque .... but then again my car is stock and will have a stock engine for another year at least so torque is not likely to show up in any sort of useful amounts

imported_MMX

Coincidently, there is an article in the Star today...

Power of a car engine is vastly overrated:
Horsepower race is misleading, since torque rules

punkindrublic

i never said either was that accurate....

just pay the $50 and dyno your car

james_tg

I agree with everyone.

punkindrublic

your forumla is good for determining what clutch to use and stuff like that, axles even on higher powered cars....

james_tg

Very true!

So many times I will see or hear of people buying turbo kits (or even nitrous) for their cars and over-looking their clutch. Needless to say, they are greeted with premature clutch failure.

What are most Civic clutches and drive shafts rated to anyways?

Nova_Dust

I must be a dumbass for asking this but what kind of car has a RPM of 90,000?

james_tg

The theoretical, meant to make a point kind

racin chick

Give me a car that digs in over a car that has 400 horsepower and no torque any day!

blk93si

H22 TORQUE

The G2 Racer

Hondas have no torque

Torque is very fun.......gives the car a little more neck snapping.

punkindrublic

f1 engines go to 18000 give or take....

i can't remember where i saw it but there was a 100cc motor that made like 45 hp at 25 000 rpm... i think it was for a go kart....

james_tg

Jesus! =) 45hp at 25'000 would sound f**ing sweet on a small go-kart

imported_BoOsTd

And this whole disscussion backs my very reason for building up a b20z vtec this winter instead of putting in another touqe less wonder b16...?? Tourque is where its at, hp is overated, most people cant tell you what tourque rateing of their engine is, but they do know their hp... High tourque and not so impressive hp rateings is a great advantage...It leaves people wondering whats wrong with their engine and why they just got smoked by a car with far less hp..

imported_gatherer

AMEN to that dude

CRXlover

I cant believe the arguments im hearing here........torque is taken and throught a mathematical calculation we get a a horsepower figure. torque is how hard the "pull" is, horsepower is kinda like how long it "pulls" for. most of the 5.7L v8s chevy has made for the past while all produce ~ 320 lbs of torque so why is the 90 z28 a 15 sec car and the newer ls1s a 13 sec car? because it has more horsepower. ie it produces its torque peak at a highter rpm and gives more avergae power throughout the rev range, in other words horsepower "wins".....sure on the street a car with more torque will be easier to launch and get an early lead but in the end all else equal the car with more horsepower wins. a b20 crx runs about a 15 sec 1/4 yet an b18c5 crx with less torque but more power will run it low to mid 13s. all else equal a car with 100hp and 200lbs trq is not going to beat the same car with 200hp and 100lbs torque. Off the line the first car will get a better launch and pull harder for a much shorter time, whereas the second car will pull less hard but pull for FAR longer. An F1 car makes 900hp@18000 rpm but only 300lbs trq... think about it if torque mattered more why would car manufacturers use horsepower ratings?

imported_JookSingKid

not true

b20crx should run 13s easy